Optically based systems for measuring a characteristic of a device under test use a material to affect optical characteristics of a light beam directed onto, into, or through the material. The material affects the optical characteristics of the beam because its index of refraction changes in response to changes in the characteristic. That change in the index of refraction causes a phase change in light passing through and/or reflection from the material; the phase change can be measured by detecting a change in the polarization state of the light.
One example of such an optically based system is an electro-optic measurement system. In an electro-optic measurement system the electro-optic material is brought close to the device under test; for some devices under test (e.g., those made of gallium arsenide (GaAs)) the device under test has electro-optic properties and can serve as the electro-optic material. Electric fields stemming from electrical effects in the device under test (and thus representative of the electrical characteristics) change the optical properties of the electro-optic material. The electro-optic material is illuminated with a measurement beam, and a beam is produced having optical characteristics affected by the electro-optic material. The affected optical characteristics of the beam are then detected and converted into an electrical signal representing the electric fields near or in the electro-optic material and thus also representing the electrical characteristics of the device under test.
Electro-optic measurement systems require precise control of the position of the electro-optic material relative to the device under test. They also require precise control of the position and alignment of the optical components associated with the measurement beam. Most work in electro-optic measurement systems has addressed those issues in a research context in which the device under test and/or the optical components associated with the measurement beam may be fixed in position for a considerable time.
In engineering or manufacturing environments it is often desirable or necessary to test electrical characteristics of a large number of devices in a time determined by economic considerations. Presently-existing electrical contact test systems for such environments provide for the rapid movement of devices to be tested through test stations. The test stations are equipped with probe arms used for two types of purposes: to apply an electrical signal to the device under test, and to carry an electrical probe for making electrical contact with a part of the device under test at which an electrical characteristic (a voltage or a current) is to be measured.
There is a need to bring the usefulness of electro-optic measurement systems into engineering or manufacturing environments. Electro-optic measurement systems offer a variety of important advantages over electrical contact test systems, Electrical contact with a device under test alters the electrical characteristics to be measured, This limits the usefulness of electrical contact systems for measuring small voltages and currents and high frequencies, Electro-optic measurement systems do not rely on electrical contact with the device under test, They offer a high bandwidth and can measure high-speed electrical characteristics such as those occurring in a time of the order of femtoseconds or with a frequency of the order of terahertz,
Electro-optic measurement systems developed in a research context present many problems which impede their adaptation for engineering or manufacturing environments, Engineering or manufacturing environments impose much greater demands for movement of the electro-optic layer relative to the device under test, Engineering environments usually must test a variety of different types of devices under test, Manufacturing environments normally must test a high volume of a single type of product and often must test more than one different type of product, In such environments the electro-optic layer must be readily and rapidly moveable with respect to the stage that holds the device under test, In such environment an electro-optic system should offer frequent three-dimensional movement of the electro-optic material relative to the device under test over a distance of the order of one centimeter, A typical operating separation between the electro-optic material and the device under test is one to five micrometers, The combination of centimeter-order movement and micrometer-order separation creates a likelihood that a user will damage the electro-optic material by contact with the device under test many times during the economic life of the electro-optic system.
Electro-optic measurement systems developed for use in a research context typically require a micropositioner to replace damaged electro-optic material with new, undamaged material. However, engineering and manufacturing environments typically lack the equipment, the skills in optics, and/or the time to carry out such replacement procedures. The process of realigning the optical elements is time-consuming even for a person trained in optics; it is difficult for a person not trained in optics. Typically persons who operate test systems in an engineering or manufacturing environment are not trained in optics. Moreover, damage to the electro-optic material necessitating replacing it may occur much more often in an engineering or manufacturing environment than in a research environment., Thus, an electro-optic system for use in an engineering or manufacturing environment must allow easy replacement of the electro-optic material by a user. Moreover, alignment of the optical components associated with the measurement beam is important for reliable operation of an electro-optic measurement system but can require considerable time and expertise. Thus, replacement of the electro-optic material should not necessitate realignment of those components.
Engineering and/or manufacturing environments also impose much greater demands for movement of the electro-optic layer relative to the device under test. Engineering environments normally must test a variety of different types of devices under test. Manufacturing environments normally must test a high volume of a single type of product and often must test more than one different type of product. In such environments the electro-optic layer must be readily and rapidly moveable with respect to a test stage that holds the device under test. Electro-optic measurement systems developed for use in a research context are not required to, and do not, have the ability provide such movement because their optical elements readily lose alignment as a result of such movement. The process of realigning the optical elements is time-consuming even for a person trained in optics; it is difficult for a person not trained in optics. Typically, persons who operate an electro-optic measurement system in an engineering or manufacturing environment are not trained in optics. Thus, an electro-optic measurement system should be able to carry out the types of movement demanded in engineering and manufacturing environments without requiring realignment of its optical systems. Moreover, the electro-optic layer must be small and readily moveable to locations near relatively complex devices under test (e.g., around raised structures or between electrical leads in integrated circuits or hybrid circuit devices). Most electro-optic systems developed for use in a research environment use microscope objectives, which are too large or cumbersome in relation to the device under test to be insertable as required.
The electro-optic systems developed in research environments do not have the:required features. There is accordingly a need for an electro-optic system that does have them.